1. Field of the Invention
The invention concerns a map display method which can be used in a geographic information system (GIS), or similar system, for example a telecommunication network optimization graphical system. A GIS is a computer tool used to organize and present spatially referenced alphanumerical data, as well as to produce drawings and maps.
2. Description of Related Art
The role of a geographic information system is to propose a more or less realistic representation based on graphic primitives such as points, vectors (arcs), polygons or grids (also known as rasters). Associated with these primitives are attribute information such as the nature of the primitive (road, railway, forest, etc.) or any other contextual information (number of inhabitants, type or surface area of a commune for example).
A geographic information system is used to handle databases for flat or three-dimensional geographic data. A geographic table consists of a traditional table (comprising tabular fields of the type string, number, Boolean, date), which is however enhanced with a specific “geographic object” field containing the value of a geo-positioned geographic object (in practice, it is a data structure comprising the type of object and the list of its X, Y and if applicable Z coordinates). Three main types of geographic objects are handled: points, polylines (pecked lines) or polygons.
The three types of objects handled each have their benefits for modeling reality and representing real objects: a point can represent a tree, an inhabitant, a site; a polyline can represent a circulation path (sewer, street, road, motorway, railway, etc.) or any kind of transmission line (high-voltage cable, pipeline). Polygons, meanwhile, can be used to mark specific regions and zones (boundaries of administrative communes, park, suburb, business park, etc.). A geographic information system is therefore used to represent the existing geographic infrastructures and environment: communes, streets, departements, regions, or even mountain ranges (in the form of datum lines).
Specific analysis features also allow classifications to be produced on a table, and objects to be automatically assigned a specific graphic display format depending on the classification values. The data associated with the objects present in the tables displayed can be read when required, by clicking on the object which appears on the map.
There are two possible representation methods:                vectorial (vector format): objects are represented by points, lines, polygons or polygons with holes;        bitmap (raster format): this is a digitized image, drawing or photo displayed in the GIS as an image.        
A system of geographic coordinates (spherical or projective) is used to reference the objects in space and position all of the objects in relation to the others. The objects are generally arranged in layers, each layer combining all homogeneous objects (building, rivers, road system, parcels, etc.).
A geographic information system is also used to represent the infrastructure data specific to a given sector. For example, a telecommunications operator may show its sites and its fiber optic links, or show mobile telephony base stations, with their coverage areas. Displaying this information in the form of a map allows the operator to understand more easily the state of the network, and therefore to manage the network to optimize the use of resources and the quality of the service.
A geographic information system offers the user a whole range of display management functions: the zoom and horizontal scroll functions are traditionally present on all geographic information systems. It is known to produce a zoom function which replaces the current image of a map with another image with the same surface area but showing a smaller (to see in greater detail) or larger (to see a larger region) land surface, depending on the choice of the user. This is the equivalent of changing the scale of the whole map displayed.
It is also known to carry out a local zoom (also known as a magnifying function) which enlarges a portion of the map displayed by displaying this portion, with a constant enlargement, above the initial image of the map, centering it over the portion to be enlarged. This method has the disadvantage of hiding a portion of the initial image of the map, the hidden surface being larger than the surface occupied by the portion to be enlarged in the initial image. For example, if the portion to be enlarged is circular and if the enlargement is +100% (on both the x and y axes), a circular image is embedded into the initial image with a radius twice that of the portion to be enlarged. As a result, the hidden surface is four times larger than the surface of the portion to be enlarged. There is therefore, around the portion which is being zoomed, a crown which is totally invisible and with a surface three times larger than the initial surface of the portion to be enlarged. This hiding of the peripheral zone can be very problematic. Furthermore, the sudden change of scale at the boundary of the enlarged portion creates a discontinuity which is highly prejudicial to the intelligibility of the objects shown.
It is known to resolve this hiding problem and this discontinuity problem through a nonlinear display method.
The thesis of T. Alan Keahey, Nonlinear Magnification, PhD thesis, Department of Computer Science, Indiana University, December 1997, describes different nonlinear display methods used to locally carry out a zoom enlarging a portion of a map, without hiding anything on the periphery of this portion, thanks to the fact that a zone located at the periphery of this portion to be enlarged is reduced, such that it compensates for the increased display surface of the portion to be enlarged.
This document also shows that such a display method can be controlled dynamically by geographic data, for example to enlarge a portion of a map showing road traffic, in order to highlight a road on which traffic measures have revealed the formation of a traffic jam, or to enlarge several portions of the same map showing air traffic, in order to highlight portions where air traffic is particularly heavy.
FIG. 1 shows a method, known by this thesis, for enlarging a circular portion in a map represented in diagram form by a checkerboard. An enlargement (enlargement value greater than 100%), roughly constant, is applied to the center of the portion to be enlarged, while a reduction (enlargement value lower than 100%), highly nonlinear, is applied to the periphery of the portion to be enlarged. More precisely, this known method involves displaying the so-called peripheral portion, located on the periphery of the portion to be enlarged, by applying respective enlargement ratios with values lower than 100% to certain elements of that peripheral portion, and such that the combination of the display surface of the peripheral portion and the display surface of the portion to be enlarged occupies, in the new image of the map, a surface equal to that of the combination of the display surface of the portion to be enlarged and the display surface of the peripheral portion in the original image of the map.
FIG. 2 shows a perspective view of a grid in which the vertical coordinate of each point represents an enlargement value used for the example of FIG. 1. A peak can be seen with a flat circular summit, and with a base surrounded by a circular valley. The vertical coordinate of the flat part surrounding this valley represents the initial enlargement (100%) of the map. The vertical coordinate of the flat summit represents the enlargement applied to the enlarged portion of the map. The valley corresponds to enlargement values lower than 100%, in other words achieving a reduction. The vertical coordinate of the bottom of the valley represents the minimum enlargement value, which corresponds to the highest reduction. On the external flank of the valley, the enlargement passes continuously from the initial value (100%) to the minimum value. On the internal flank, the enlargement passes progressively from the minimum value to the initial value (100%) constituting the base of the peak.
FIG. 3 shows a perspective view of the same grid, superimposed with the map represented in diagram form by a checkerboard.
FIG. 4 shows an actual map, from the Washington subway network, on which such a known method has been applied to enlarge a circular portion located at the center of the map. The objects located at the periphery of the enlarged portion are reduced. This figure shows that there is no hidden zone, and no discontinuity of the objects represented. A slight blur is applied to the peripheral zone to mark the boundary between the enlarged portion and the non-enlarged portion of the map. It should be noted that, despite the absence of hiding, there is a loss of information in the peripheral zone since the screen and the eye do not have an infinite resolution ability: the subway stations located in the peripheral zone are visible, however their names are not legible due to the distortion and the reduction in the size of the characters.
The document HONGZHI SONG ET AL: “LensList: Browsing and Navigating Long Linear Information Structures HUMAN INTERFACE AND THE MANAGEMENT OF INFORMATION. METHODS, TECHNIQUES AND TOOLS IN INFORMATION DESIGN; [LECTURE NOTES IN COMPUTER SCIENCE], SPRINGER BERLIN HEIDELBERG, BERLIN, HEIDELBERG, vol. 4557, 22 Jul. 2007 (2007 Jul. 22), pages 535-543, XP019064238 ISBN: 978-3-540-73344-7* FIGS. 1-3, pages 537-539* describes a method for displaying a list of words (constituting a menu for example), which provides a magnifying effect on a central portion of this list, without hiding the upper portion or the lower portion of this list, thus guaranteeing the legibility of all the words of the list. It involves enlarging the font of the words in the central portion to be enlarged and reducing the font of the words in the upper portion and in the lower portion. The size of the font increases linearly from the top of the list displayed towards the center of the list displayed, then decreases linearly from the center towards the end of the list displayed. The maximum size and the minimum size are chosen by the user.
A disadvantage of the known methods is that all the objects located in the peripheral portion surrounding the enlarged portion have a reduced size. Choosing the correct enlargement in this peripheral portion can ensure the legibility of all the objects located in this peripheral portion. However, an important object located in the peripheral portion may pass unnoticed since the legibility is reduced. This may be serious if this object is a symbol indicating an alarm, or any other event which may appear unpredictably.